Gas discharge tubes (GDT) are well known for their use in providing switching to shunt an applied over-potential to a voltage reference, such as ground. The gas discharge tube operates to protect associated equipment by preventing voltage signals above a predetermined level from affecting the equipment. The tubes find use in telephone and cable television systems. In the latter, the signal voltage is carried by a coaxial cable. The gas discharge tube provides over-voltage protection and is a failsafe mechanism that provides protection against gas tube failure, such as by gas venting, and thermal overload.
Three element gas discharge tubes are in common use. These tubes have, what is commonly called, tip (or "a"), ring (or "b") and ground electrodes. When the "a" or "b" electrode of the three element tube receives a voltage equal to or greater than the tube breakdown voltage, it causes the tube gas to ionize, or break down, and the tube to conduct to provide the required short circuit to ground for voltage overload protection. Two element gas discharge tubes also are used in which one tube electrode receives the applied voltage and the other electrode is connected to ground. These operate such that the gas in the tube ionizes when the signal voltage exceeds a predetermined level as set by the tube gas and tube dimensions.
Both two element and three element gas discharge tubes are subject to possible failure, for example, breakage of the tube that permits the gas to vent. If this is unknown to the system operator, there will be no over-voltage protection. Another problem is thermal overload of the gas discharge tube, which makes it inoperative, and may cause a fire hazard.
Protection, that is, backup, against tube failure is often required. For example, in the case of tube gas venting and the consequent tube failure, there is a requirement for a backup gap device that will provide over-voltage protection in response to a certain value of applied voltage. In addition, thermal overload protection is also required to protect against repeated firing in power-cross scenarios.